John Milbank, Research Professor of Politics, Religion and Ethics at the University of Nottingham, Director of the Centre of Theology and Philosophy, and Chairman of the ResPublica Trust argues in a lengthy essay for the Australian Broadcasting Company's "Religion and Ethics" section that redefining marriage obscures the nature of marriage fundamentally and irrevocably.
Here's one of his more compelling arguments (of which he makes many):
...Increasingly, children resulting from anonymous artificial insemination are rightly demanding to know who their natural parents are - for they know that, in part, we indeed are our biology. But on the other hand, this request is in principle intolerable for donors who gave their sperm or wombs on the understanding that this was an anonymous donation for public benefit -- like blood donation properly precluding any personal involvement.
The recipe for psychological confusion, family division and social conflict involved here is all too evident and cannot be averted. In this instance we have sleep-walked into the legalisation of practices whose logic and implications have never been seriously debated.
From this it follows that we should not re-define birth as essentially artificial and disconnected from the sexual act - which by no means implies that each and every sexual act must be open to the possibility of procreation, only that the link in general should not be severed.
The price for this severance is surely the commodification of birth by the market, the quasi-eugenic control of reproduction by the state, and the corruption of the parent-child relation to one of a narcissistic self-projection.
Once the above practices have been rejected, then it follows that a gay relationship cannot qualify as a marriage in terms of its orientation to having children, because the link between an interpersonal and a natural act is entirely crucial to the definition and character of marriage.
The fact that this optimum condition cannot be fulfilled by many valid heterosexual marriages is entirely irrelevant, for they still fulfil through ideal intention this linkage, besides sustaining the union of sexual difference which is the other aspect of marriage's inherently heterosexual character.